It seems like the chemtrails conspiracy theory pops up every so often but the BBC has a nice write-up of what contrails are and why they occur.

Contrails are formed when water vapour and fine soot particulates from burning jet fuel freeze into ice crystals. In low air humidity, the crystals just dissipate. In higher humidity, they persist, and end up creating visible vapour trails over large areas of sky.

Adding to this, even if it’s humid, you may only see thin clouds because the upper level winds blow the contrails apart.

Oddly enough, the conspiracy theory likely gets a bit of weight behind because of experiments in the 1950s and 1960s known as the Dorset Biological Warfare Experiments. Similar experiments took place in the United States and in 1977 the US Senate Subcommittee on Health and Scientific Research led to the US Army revealing the details of the experiments.